Because "the fake thing" probably wouldn't sell a lot of Coke. Also, to subtlely suggest Coca Cola to be separate and superior to other products with a similar name, ie Pepsi Cola and RC Cola.
Coke
Examples of a slogan include 'The happiest place on Earth.' used by Disneyland and 'It's the real thing.' used by Coca Cola. A slogan is a memorable phrase used to express a purpose or an idea.
Let me manifest, if you want to live, digest! And yes, that's not a real slogan.
You Know When It's Real or Chewy and Moist.
The real coffee experts.
If Xerox Corporation can be said to have a business slogan, it is probably "Ready for real business."
TONKA
I'm lovin' it You know when it's real
Dont give in, we want the real you to win!
The slogan "Building India" belongs to a company called DLF, which is the largest real estate company in India.
Gotland --FreedomFalling HI2
Where homes click! But, you have to brand yourself as a .com company.